By taking advantage of a homeworld mission, you can purchase these for only 1,000. To do this, take the "I want to Terraform" mission. Use the Atmosphere Generator anywhere, except for the target planet. Return to your homeworld and go through the mission dialog until you get two options. Choose "I need more of the tools you gave me." You will be given a new Atmosphere Generator along with 10 more of the plants and creatures at the cost of 1,000.

Trivia

The atmosphere generator somewhat resembles the atmosphere processor from the movie Aliens, which was what was producing breathable oxygen for the human colonists on LV 426.